Update – the article has been rewritten based on my own tests, since there are conflicting reports elsewhere of whether 86Mbit is achievable or not, let alone reliable.
The Panasonic GH1 firmware hack has now been around for some time although new parameters are being added for testers by Vitaily Keslev.
One of the parameters is labeled ‘overall bitrate’ for the GH1’s AVCHD encoder. However since it is not yet fully understood by testers how each AVCHD encoder parameter directly effects the encoder and how combinations of settings work in unison, a simple change to the ‘overall bitrate’ is not as simple as changing one settings.
The GH1 uses a variable bitrate encoder to produce it’s AVCHD footage. This means that should the amount of detail in the scene require it, the bitrate increases on the fly.
Based on my own personal testing the ‘overall bitrate’ parameter appears to effect the card controller’s burst rate, the peak bitrate at which the card controller can write a AVCHD stream to the SD card. If the bitrate increases too high for a very detailed frame, and if the multitude of settings on PTools allows it to increase beyond a certain point, the camera crashes due to the limits of it’s hardware – in terms of buffer memory, encoder CPU and card I/O controller.
The overall bitrate setting may determine an upper limit for which the card I/O controller must be set, so not to crash the camera. Panasonic needed such settings to maintain the reliability of the camera even with cheap SD cards which do not have good sustainable write speeds.
However most fast SD cards are capable of much higher write speeds than 86Mbit AVCHD video demands and here lies the mystery.
There doesn’t currently seem to be a way to stop the camera crashing with such high bitrates. More parameters are required and testers are almost blind to the inner workings of the GH1’s firmware code. The reasons for the crashes are currently quite hard to determine.
Although the camera’s hardware appears very capable, my own tests have found that 86Mbit is not reliable and that the actual average bitrate is usually much lower, even for scenes with a reasonably high level of detail – the encoder rarely goes beyond 40Mbit.
I currently have a setting which I’m very happy with – it comes to a point where it is not worth chasing higher bitrates and compromising reliability, because the gains tail off the higher the bitrate goes.
Even 24Mbit is fine if the AVCHD encoder is well implemented.
The best thing to do is to experiment with PTools and shoot footage over and over until you find the setting that works for you and is reliable.
Research is ongoing but I am turning off to it, because I do not feel there is anything more to come from the hack – in my view image quality gains have plateaued and the image quality is already great at 44Mbit, plus we have the GH2 to look forward to.